An electric motor is formed of a rotor and a stator. When the temperature of the rotor of the electric motor becomes high, heat transfers to the driven body so that various kinds of adverse influence such as thermal expansion of the driven body are feared to take place. Further, as to a rotor using permanent magnets, there is a possibility that the permanent magnets might be demagnetized at high temperature. Therefore, the control device of the electric motor is demanded to have a function of preventing overheat of the rotor.
As a method of preventing overheat of the rotor of an electric motor, there has been a known method in which the temperature of the rotor of the electric motor is estimated from the rotational speed and current of the electric motor (e.g., Japanese Patent 5149431 (JP 5149431B), which will be referred to hereinbelow as “Patent Document 1”). The above invention is roughly divided into two temperature estimation methods.    (i) Core loss is estimated and the estimated core loss is used to estimate the rotor temperature.    (ii) The rotor temperature is estimated by taking into account the heat transfer from the stator in addition to the core loss.
Herein, as explanation (ii), the temperature estimation formula is defined as follows:
            T      r        ⁡          (              t        +                  Δ          ⁢                                          ⁢          t                    )        =                    T        r            ⁡              (        t        )              +                                        p            ⁢                          (              t              )                                +                                    k              1                        ⁡                          (                                                                    T                    c                                    ⁡                                      (                    t                    )                                                  -                                                      T                    r                                    ⁡                                      (                    t                    )                                                              )                                -                                    k              2                        ⁡                          (                                                                    T                    r                                    ⁡                                      (                    t                    )                                                  -                                  T                  s                                            )                                      C            ⁢      Δ      ⁢                          ⁢      t      where,
t, Δt, t+Δt: time at which AC current is applied to the electric motor;
Tr(t): rotor temperature at time t;
Tr(t+Δt): rotor temperature at time (t+Δt)
Tc(t): winding temperature at time t;
p(t): core loss at time t;
TS: ambient temperature around the electric motor; and,
k1, k2, C: constants determined in accordance with the shape and material of the electric motor and the cooling condition of the stator.
As in the above formula, the equation is complicated, and therefore this method has the problem that not only each coefficient in the formula is difficult to determine but also the validity of each coefficient is hard to confirm.
On the other hand, as explanation (i), the temperature estimation formula is defined as follows:
                                          T            r                    ⁡                      (                          t              +                              Δ                ⁢                                                                  ⁢                t                                      )                          =                                            T              r                        ⁡                          (              t              )                                +                                                                      p                  ⁢                                      (                    t                    )                                                  -                                  k                  ⁡                                      (                                                                                            T                          r                                                ⁡                                                  (                          t                          )                                                                    -                                              T                        s                                                              )                                                              C                        ⁢            Δ            ⁢                                                  ⁢            t                                              (        1        )            where,
t, Δt, t+Δt: time at which AC current is applied to the electric motor;
Tr(t): rotor temperature at time t;
Tr(t+Δt): rotor temperature at time (t+Δt);
p(t): core loss at time t;
TS: ambient temperature around the electric motor; and,
k, C: constants determined in accordance with the shape and material of the electric motor and the cooling condition of the stator.
Determination of coefficients and confirmation of their validity are easier in (i) compared to (ii), but (i) has the problem that the estimation accuracy is lower than that of (ii) because no consideration on heat transfer from the stator is given.
The reason why the estimation accuracy in the method (i) lowers will be described. It is possible to simplify the estimation formula of the core loss by flowing D-phase current so as to minimize eddy current while regarding that the hysteresis loss is sufficiently small. Specifically, in the core loss estimation formula in the prior art (see Patent Document 1) shown below:p={a|Iq|α+b|c+Id|α}ω2+{e|Iq|β+f|c+Id|β}ωwhen Id=−c, e=0 and f=0, the core loss estimation formula can be written as follows:p=a|Iqαω2 
Further, the index α may be empirically set at 2, and when taking into account that the Q-phase current Iq is proportional to torque and that the power of the electric motor can be obtained by multiplying torque by the rotational speed, the core loss after simplification can be represented as follows:p=kP2  (2)where
P: power of the electric motor, and,
k: a proportionality coefficient.
On the basis of the above, it is possible to consider a method of continuously estimating the temperature of the rotor by monitoring the output power (or Q-phase current×rotational speed) of the electric motor.
However, when the temperature of the stator is high and hence a large amount of heat transfers from the stator to the rotor, it can be said that temperature estimation of the rotor based only on the power of the electric motor exposes a lower accuracy compared to the above method (ii).
The above problem will be explained by providing a specific example. A comparison is made between when the rotational speed is ω with torque of 2T and when the rotational speed is 2ω with torque of T. In both cases, the output power is 2Tω, hence the estimated core losses are the same. However, in the case with a rotational speed of ω, double torque is generated compared to the case with a rotational speed of 2ω. Therefore, doubled Q-phase current must flow. Accordingly, when considering copper loss of the stator, the copper loss at a rotational speed of ω becomes greater than the copper loss at a rotational speed of 2ω. Accordingly, the stator temperature (windings temperature) becomes high so that a greater amount of heat transfers from the stator to the rotor. As a result, it can be said that in the condition with the same power and the same core loss, the lower the speed the higher the actual temperature of the rotor becomes though the estimated temperature of the rotor is the same.
A more detailed explanation will be given as to the relation between the estimated rotor temperature and the actual rotor temperature when the electric motor is driven at a low rotational speed and at a high rotational speed. FIGS. 1A and 1B are graphs representing the estimated temperature and the actual temperature of a rotor when an electric motor is driven at a low rotational speed and at a high rotational speed, respectively. As depicted in FIG. 1A it is considered that when the electric motor is driven at a low rotational speed the actual temperature of the rotor becomes higher than the estimated temperature. On the other hand, as depicted in FIG. 1B it is considered that when the electric motor is driven at a high rotational speed the actual temperature of the rotor becomes lower than the estimated temperature. In this way, it can be said that at the lower speed the electric motor is driven, the higher the actual temperature of the rotor becomes.
It is therefore an object of the present invention to provide a temperature detecting device of the rotor in an electric motor and an overheat protection device of an electric motor that can achieve improved accuracy of temperature estimation by simply determining a coefficient for calculating the estimated temperature of the rotor.